Improving the fuel economy and safety of passenger jet aircraft continues to be very important to operating competitively in today's crowded transportation marketplace. Reducing fuel consumption while maintaining the same or similar levels of service lowers operating costs, consumes fewer resources and, in some cases, may extend the capabilities of the aircraft.
In the case of non-redundant two-engine passenger jet aircraft, there is a class of long routes, referred to as “extended operations” (ETOPS) routes, that have conventionally required an auxiliary power unit to be run continuously (“Continuous Run”), in addition to the operation of both engines on 737 aircraft. An auxiliary power unit (APU) is a small gas turbine engine used to supply pneumatic power for various subsystems of the aircraft including starting the main turbine engines, as well as supplying electrical power. If the APU is on Continuous Run status (APUCR), the APU consumes fuel and wear and tear on the APU is increased, thus detracting from the overall efficiency of the aircraft. By contrast, if the APU is not run continuously, or is only run when needed, which is referred to as “On Demand” (APUOD) status, the APU consumes far less fuel and wear and tear is reduced, thus improving the overall economy of the aircraft.
It would be desirable to provide a robust approach for operating the APU only when required, such as to provide backup electrical in the event of a generator, or an engine failure.